1. Field of the Invention
The present invention is generally related to a nuclear reactor fuel assembly and in particular to a bottom nozzle plate used with the fuel assembly.
2. General Background
Commercial nuclear reactors include multiple fuel assemblies. Each fuel assembly is comprised of a number of fuel rods radially spaced apart in a parallel array by grid assemblies spaced along the length of the fuel rods. Each grid assembly is formed in an eggcrate design by multiple metal strips that criss-cross at right angles to form individual cells for each of the fuel rods. The strips are provided with tabs that project into the cells against the fuel rods. The tabs serve the purpose of holding the fuel rods in their respective radial position. Control rod guide thimble tubes also extend through selected cells in the grid assembly and are attached at their upper and lower ends respectively to an upper end fitting and a lower end fitting. The upper and lower end fittings are also commonly referred to in the industry as nozzle plates since they are rigid plates that provide structural integrity and load bearing support to the fuel assembly and are provided with flow apertures therethrough for coolant flow. The lower end fitting or nozzle plate is positioned directly above openings in the lower portion of the reactor where coolant flows up into the reactor to the core. The ligaments between apertures in the end fittings coincide with the ends of the fuel rods and limit upward or downward movement of the fuel rods. Debris such as metal particles, chips, and turnings is generated during manufacture, installation, and repair of the reactor, piping, and associated cooling equipment. The size and complexities of the equipment prevent location and removal of all such debris before operations are commenced. Also, some of this debris may not become loose matter in the system until the system is put into operation. It has been recognized that this debris presents a greater problem to the system than previously thought. These small pieces of debris have been found to lodge between the walls of the grid cells and the fuel elements. Movement and vibration of the lodged debris caused by coolant flow results in abrasion and removal of cladding on the fuel rods. This in turn leads to detrimental effects such as corrosion of the fuel rods and failure to retain radioactive fission gas products. Such damage, although not critical to safety of the surrounding environment, can reduce operating efficiency by the need to suspend operation while replacing damaged fuel rods. It can be seen that a need exists for a debris filter capable of filtering debris of a size which may lodge between the grid cell walls and the fuel rods. An important consideration besides that of filtration is that a substantial coolant pressure drop across the filter must be avoided in order to maintain an adequate coolant flow over the fuel rods for heat removal therefrom. Patented approaches to this problem of which applicant is aware include the following.
U.S. Pat. Nos. 4,684,495 and 4,684,496 disclose debris traps formed from a plurality of straps aligned with one another in a crisscross arrangement and defining a plurality of interconnected wall portions which form a multiplicity of small cells each having open opposite ends and a central channel for coolant flow through the trap.
U.S. Pat. No. 4,828,791 discloses a debris resistant bottom nozzle which is a substantially solid plate having cut-out regions in alignment with inlet flow holes in the lower core plate. Separate criss-cross structures fixed to the plate extend across the cut-out regions to act as a debris trap.
U.S. Pat. Nos. 4,664,880 and 4,678,627 disclose debris traps mounted within a bottom nozzle that define a hollow enclosure with an opening so as to form a debris capturing and retaining chamber.
U.S. Pat. No. 4,652,425 discloses a trap for catching debris disposed between the bottom nozzle and the bottom grid. The structure forms multiple hollow cells that receive the fuel rod lower end plugs with dimples in each cell for catching debris carried into the cells by the coolant flow.
U.S. Pat. No. 4,900,507 discloses a nuclear fuel assembly debris filter bottom nozzle formed from a rigid plate having a plurality of flow holes. The holes are individually 0.190.+-.0.008 inch in diameter which is smaller than the predetermined maximum cross-sectional dimension of each of the unoccupied spaces through the lowermost grid. The sections of the plate extending between the flow holes have a maximum dimension of one-tenth inch.
Although a variety of approaches have been taken to the need for filtration of primary coolant, it is felt that the problem has not been adequately addressed. While certain designs such as those utilizing only small flow holes may provide adequate filtration of damaging debris, the problem of an unacceptable pressure drop may still exist.